SAE International Periodic Reverse Flow and Boiling Fluctuations in a Microchannel Evaporator of an R134a Mobile Air-Conditioning System 2013-01-1500

This paper presents experimental study of periodic reverse flow and induced boiling fluctuations in a microchannel evaporator and their impacts on performance of R134a mobile A/C system. Simultaneous flow visualization and pressure measurements revealed that reverse flow due to confined bubble longitudinal expansion caused periodic oscillations of the evaporator inlet pressure and the pressure drop, and their oscillation magnitude and frequency increase with ambient air temperatures because of higher average refrigerant mass flux and heat flux. Three potential impacts of vapor reverse flow reversal on evaporator performance are identified: 1) mild liquid maldistribution; 2) increased the evaporator pressure drop; 3) reduced heat transfer coefficient. Finally, to mitigate vapor reverse flow impacts, revised flash gas bypass (FGBR) method is proposed: vent and bypass backflow vapor trapped in the inlet header. Experimental results showed that system performances are further improved by an up to 5% higher capacity and an up to 3% higher COP.